6ACTIVITY

background

A wide range of organic molecules, both naturally occurring and man-made, suchas therapeutic medicines and dyes tend to be aromatic molecules. In this activitystudents will carry out the nitration of a simple aromatic compound and then lookat the product using various analytical techniques.

.

pre-planning required

weeks beforeBook the laboratory and technicians. Organise demonstrators, book the nuclearmagnetic resonance (NMR) spectrometer, mass spectrometer and infraredspectroscopy (IR) facilities, and order the chemicals.

days beforeGet the final list of students attending and divide then into groups. It works best ifthe schools are spread within groups and is probably most efficient if the studentswork in pairs. Print the copies of spectra from the ChemSoc website database,www.chemsoc.org/networks/learnnet/spectra/index2.htm

• concentrated sulfuricacid

• concentrated nitric acid • methyl benzoate• deionised water• crushed ice • ice baths• thermometers • dropping pipettes (eg

'calibrated' plastic)• Buchner flasks • Buchner funnels and

suitable sized filter paper • ethanol • steam baths • melting point apparatus• conical flasks • beakers • stirring rods • water bottles• large filter papers

materials required

Appropriate lecture theatre and laboratory

facilities required

This resource is based on an activity run by Dr Alethea Tabor, University College, London.

09.45 Arrival10.15 Lecture on chirality and its

importance in chemistry, biologyand pharmacology

10.45 Lecture looking at the discoveryof new medicines

11.30 Break11.45 Careers lecture – why study

chemistry?12.30 Lunch13.15 Organic practical session14.00 Groups are taken to analyse the

Suggested timings for the day

starting material by either MS,NMR or IR. Try to finish by 14.45.

14.50 Groups are taken to analyse theirproduct by either MS, NMR or IR.

15.10 Another group is taken toanalyse the product usingmass spectrometry.

15.45 Break16.00 Lecture on up to date research

being done at the university17.00 Finish

organicmolecules

day

Copyright © 2007 Royal Society of Chemistry www.rsc.org/outreach

DE

SIG

NE

DA

ND

PR

OD

UC

ED

BY

SA

GE

AS

SO

CIATE

SW

WW

.SA

GE

AS

SO

CIATE

S.C

O.U

K

A riskassessment must bedone for this activity.

SAFETY

P6.1 student worksheet

organic molecules day

introduction

This activity is designed to give you an insight into what organic chemists do intheir day to day work.

background

A wide range of organic molecules, both naturally occurring and man-made, suchas therapeutic medicines and dyes are aromatic compounds. The simplestaromatic compound is benzene.

Aromatic compounds normally undergo substitution reactions – rather than additionreactions. This is because substitution reactions retain the delocalised -electronsystem of the aromatic nucleus.

One of the most important substitution reactions of aromatic compounds isnitration – ie the addition of an NO2 group.

The experiment

In this experiment you will nitrate methylbenzoate, which is a relatively simple non-toxic derivative of benzene. You will use amixture of nitric acid and sulfuric acid tocarry out the nitration and then purify theproduct by recrystallisation.

Finally you will determine the structure ofthe nitro compound that you have made.How many nitro groups you have added tothe ring, and where? You will use a rangeof spectroscopic techniques – massspectrometry (MS), infrared (IR)spectroscopy and nuclear magneticresonance (NMR) spectroscopy – to workout the answer.

The procedure

Measure 2.5 cm3 of methyl benzoateinto a small conical flask and then dissolveit in 5 cm3 of concentrated sulfuric acid.When the liquid has dissolved, cool themixture in ice.

Prepare the nitrating mixture bycarefully adding 2 cm3 of concentrated

H

H

H H

H

H

H

C

CC

C

C

CH

H H

H

H

H-H

+E+

EE

C

O

O CH3

CH3

C

O

O

Copyright © 2007 Royal Society of Chemistry www.rsc.org/outreach

Laboratory coatsand gloves must be worn atall times. Concentratedsulfuric acid andconcentrated nitric acidmust be measured in a fumecupboard. No naked flamesshould be present whileethanol is being used.

SAFETY

sulfuric acid to 2 cm3 of concentrated nitricacid. Cool this mixture in ice.

Add the nitrating mixture – drop bydrop from a teat pipette – to the solution ofmethyl benzoate. (NB: Do not allow thenitrating mixture to get into the rubber teat.)Stir the mixture with a glass rod and keepthe temperature below 10 ºC. When youhave completed the addition, leave themixture to stand at room temperature foranother 15 minutes.

After 15 minutes, pour the reactionmixture on to about 25 g of crushed iceand stir until all the ice has melted and thecrystalline nitro derivative forms.

Filter the crystals using a Buchnerfunnel, wash thoroughly with cold waterand then transfer to a small beaker.

Recrystallise the product from hotethanol. The idea of recrystallisation is todissolve the impure product in theminimum possible volume of hot solvent.When you cool the solution, the productthat you want crystallises out of solution.This is because there is not enough solventto dissolve it all at the lower temperature.However, the impurities stay behind insolution, because there are less of these

impurities there is enough solvent to keepthem dissolved. You then filter off thecrystals of the product from the remainingsolution.

Put 15 cm3 of ethanol in a boiling tube.Warm it to about 50 °C on the steam bath.Dissolve all the crystals in the minimumpossible volume of this hot ethanol. Allowthe solution to cool to room temperature,then immerse the beaker in ice to completethe crystallisation.

Filter the crystals and dry thembetween filter paper. If there is time,measure the melting point of the crystals.

student worksheet 6.2P

vacuum

Copyright © 2007 Royal Society of Chemistry www.rsc.org/outreach

P6.3 student worksheet

Copyright © 2007 Royal Society of Chemistry www.rsc.org/outreach

DE

SIG

NE

DA

ND

PR

OD

UC

ED

BY

SA

GE

AS

SO

CIATE

SW

WW

.SA

GE

AS

SO

CIATE

S.C

O.U

K

Analysis

Infrared spectroscopy Compare thespectra of the methyl benzoate and theproduct. What are the differences? Whatcan you deduce?

Mass spectrometry Compare thespectra of the methyl benzoate and theproduct. How many nitro groups have beenadded to the benzene ring? Can you saywhat position(s) have been substituted?

Nuclear magnetic resonancespectroscopy Compare the spectra ofthe methyl benzoate and the product.Can you tell how many nitro groups havebeen added? Can you work out where?Are there any impurities present?Which technique do you find most usefuland why?

The theory of these analytical techniquesis available on a separate sheet.